Chapter 6

Iteration

6.1 Multiple assignment

As you may have discovered, it is legal to
make more than one assignment to the same variable. A
new assignment makes an existing variable refer to a new
value (and stop referring to the old value).

bruce = 5
print bruce,
bruce = 7
print bruce

The output of this program is 5 7, because the first time
bruce is printed, his value is 5, and the second time, his
value is 7. The
comma at the end of the first print statement suppresses
the newline after the output, which is why both outputs
appear on the same line.

Here is what multiple assignment looks like in a state diagram:

With multiple assignment it is especially important to distinguish
between an assignment operation and a statement of equality. Because
Python uses the equal sign (=) for assignment, it is tempting to
interpret a statement like a = b as a statement of equality. It
is not!

First, equality is commutative and assignment is not. For
example, in mathematics, if a = 7 then 7 = a. But in Python, the
statement a = 7 is legal and 7 = a is not.

Furthermore, in mathematics, a statement of equality is always true.
If a = b now, then a will always equal b. In Python, an
assignment statement can make two variables equal, but they don't have
to stay that way:

a = 5
b = a # a and b are now equal
a = 3 # a and b are no longer equal

The third line changes the value of a but does not change the
value of b, so they are no longer equal. (In some
programming languages, a different symbol is used for assignment,
such as <- or :=, to avoid confusion.)

Although multiple assignment is frequently helpful, you should use it
with caution. If the values of variables change frequently, it can
make the code difficult to read and debug.

6.2 The while statement

Computers are often used to automate repetitive tasks. Repeating
identical or similar tasks without making errors is something that
computers do well and people do poorly.

We have seen two programs, nLines and countdown, that use
recursion to perform repetition, which is also called iteration.
Because iteration is so common, Python provides several language
features to make it easier. The first feature we are going to look
at is the while statement.

Here is what countdown looks like with a
while statement:

defcountdown(n):
while n > 0:
print n
n = n-1
print"Blastoff!"

Since we removed the recursive call, this function is not
recursive.

You can almost read the while statement as if it were English.
It means, "While n is greater than 0, continue
displaying the value of n and then reducing the value of
n by 1. When you get to 0, display the word Blastoff!"

More formally, here is the flow of execution for a while statement:

Evaluate the condition, yielding 0 or 1.

If the condition is false (0), exit the while statement
and continue execution at the next statement.

If the condition is true (1), execute each of the statements in the
body and then go back to step 1.

The body consists of all of the statements below the header
with the same indentation.

This type of flow is called a loop because the third step
loops back around to the top. Notice that if the condition is false
the first time through the loop, the statements inside the loop are
never executed.

The body of the loop should change the value of one or more variables
so that eventually the condition becomes false and the loop
terminates. Otherwise the loop will repeat forever, which is called
an infinite loop. An endless source of amusement for computer
scientists is the observation that the directions on shampoo,
"Lather, rinse, repeat," are an infinite loop.

In the case of countdown, we can prove that the loop
terminates because we know that the value of n is finite, and we
can see that the value of n gets smaller each time through the
loop, so eventually we have to get to 0. In other
cases, it is not so easy to tell:

The condition for this loop is n != 1, so the loop will continue until
n is 1, which will make the condition false.

Each time through the loop, the program outputs the value of n
and then checks whether it is even or odd. If it is even, the value
of n is divided by 2. If it is odd, the value is replaced by
n*3+1. For example, if the starting value (the argument passed
to sequence) is 3, the resulting sequence is 3, 10, 5, 16, 8, 4, 2, 1.

Since n sometimes increases and sometimes decreases, there is no
obvious proof that n will ever reach 1, or that the program
terminates. For some particular values of n, we can prove
termination. For example, if the starting value is a power of two,
then the value of n will be even each time through the loop
until it reaches 1. The previous example ends with such a sequence,
starting with 16.

Particular values aside, the interesting question is whether we can
prove that this program terminates for all positive values of n.
So far, no one has been able to prove it or disprove it!

As an exercise, rewrite the function nLines from
Section 4.9 using iteration instead of recursion.

6.3 Tables

One of the things loops are good for is generating tabular data.
Before computers were readily available, people had to calculate
logarithms, sines and cosines, and other mathematical functions
by hand. To make that easier, mathematics books contained long tables
listing the values of these functions. Creating the tables was
slow and boring, and they tended to be full of errors.

When computers appeared on the scene, one of the initial reactions
was, "This is great! We can use the computers to generate the tables,
so there will be no errors." That turned out to be true (mostly) but
shortsighted. Soon thereafter, computers and calculators were so
pervasive that the tables became obsolete.

Well, almost. For some operations, computers use
tables of values to get an approximate answer and then perform
computations to improve the approximation. In some cases, there have
been errors in the underlying tables, most famously in the table the
Intel Pentium used to perform floating-point division.

Although a log table is not as useful as it once was, it still makes
a good example of iteration. The following program outputs a sequence
of values in the left column and their logarithms in the right column:

x = 1.0
while x < 10.0:
print x, '\t', math.log(x)
x = x + 1.0

The string
'\t' represents a tab
character.

As characters and strings are displayed on the screen,
an invisible marker called the cursor keeps track of
where the next character will go. After a print statement, the
cursor normally goes to the beginning of the next line.

The tab character shifts the cursor to the right until it
reaches one of the tab stops. Tabs are useful for making columns of
text line up, as in the output of the previous program:

If these values seem odd, remember that the log function uses
base e. Since powers of two are so important in computer
science, we often want to find logarithms with respect to base 2. To
do that, we can use the following formula:

Now instead of adding something to x each time through the loop, which
yields an arithmetic sequence, we multiply x by something, yielding a
geometric sequence. The result is:

1.0 0.0
2.0 1.0
4.0 2.0
8.0 3.0
16.0 4.0
32.0 5.0
64.0 6.0

Because of the tab characters between the columns, the position of the
second column does not depend on the number of digits in the first
column.

Logarithm tables may not be useful any more, but for computer
scientists, knowing the powers of two is!

As an exercise, modify this program so that it outputs the powers
of two up to 65,536 (that's 216). Print it out and memorize it.

The backslash character in '\t' indicates the
beginning of an escape sequence. Escape sequences
are used to represent invisible characters like
tabs and newlines. The sequence \n represents a newline.

An escape sequence can appear
anywhere in a string; in the example, the tab escape
sequence is the only thing in the string.

How do you think you represent a backslash in a string?

As an exercise, write a single string that

produces
this
output.

6.4 Two-dimensional tables

A two-dimensional table is a table where you
read the value at the intersection of a row and a column. A
multiplication table is a good example.
Let's say you want to print a multiplication table for the values
from 1 to 6.

A good way to start is to write a loop that prints the multiples of
2, all on one line:

i = 1
while i <= 6:
print 2*i, ' ',
i = i + 1
print

The first line initializes a variable named i, which acts as a
counter or loop variable. As the loop executes, the value of
i increases from 1 to 6. When i is 7, the loop
terminates. Each time through the loop, it displays the value of 2*i, followed by three spaces.

Again, the comma in the print statement suppresses the newline.
After the loop completes, the second print statement starts a
new line.

The output of the program is:

2 4 6 8 10 12

So far, so good. The next step is to encapsulate and generalize.

6.5 Encapsulation and generalization

Encapsulation is the process of wrapping a piece of code in a
function, allowing you to take advantage of all the things functions
are good for. You have seen two examples of encapsulation:
printParity in Section 4.5; and
isDivisible in Section 5.4.

Generalization means taking something specific, such as printing the
multiples of 2, and making it more general, such as printing the
multiples of any integer.

This function encapsulates the previous loop and
generalizes it to print multiples of n:

6.6 More encapsulation

To demonstrate encapsulation again, let's take the code from the end of
Section 6.5 and wrap it up in a function:

defprintMultTable():
i = 1
while i <= 6:
printMultiples(i)
i = i + 1

This process is a common development plan. We develop code by
writing lines of code outside any function, or typing them in to the
interpreter. When we get the code working, we extract it and wrap it
up in a function.

This development plan is particularly useful if you don't know, when
you start writing, how to divide the program into functions. This
approach lets you design as you go along.

6.7 Local variables

You might be wondering how we can use the same variable, i, in
both printMultiples and printMultTable. Doesn't it cause
problems when one of the functions changes the value of the variable?

The answer is no, because the i in printMultiples and the
i in printMultTable are not the same variable.

Variables created inside a function definition are local; you can't
access a local variable from outside its "home" function. That
means you are free to have multiple variables with the same name as
long as they are not in the same function.

The stack diagram for this program shows that the two
variables named i are not the same variable. They can refer to
different values, and changing one does not affect the other.

The value of i in printMultTable goes from 1 to 6. In the
diagram it happens to be 3. The next time through the loop it will
be 4. Each time through the loop, printMultTable calls
printMultiples with the current value of i as an
argument. That value gets assigned to the parameter n.

Inside printMultiples, the value of i goes from
1 to 6. In the diagram, it happens to be 2. Changing this variable
has no effect on the value of i in printMultTable.

It is common and perfectly legal to have different local variables
with the same name. In particular, names like i and j are
used frequently as loop variables. If you avoid
using them in one function just because you used them somewhere else,
you will probably make the program harder to read.

6.8 More generalization

As another example of generalization, imagine you wanted a program
that would print a multiplication table of any size, not just the
six-by-six table. You could add a parameter to printMultTable:

defprintMultTable(high):
i = 1
while i <= high:
printMultiples(i)
i = i + 1

We replaced the value 6 with the parameter high. If we call
printMultTable with the argument 7, it displays:

This is fine, except that we probably want the table to be
square with the same number of rows and columns. To do that, we
add another parameter to printMultiples to specify how many
columns the table should have.

Just to be annoying, we call this parameter high, demonstrating
that different functions can have parameters with the same name (just like
local variables). Here's the whole program:

When you generalize a function appropriately, you often get
a program with capabilities you didn't plan. For
example, you might
notice that, because ab = ba,
all the entries in the table appear twice. You could save ink by printing
only half the table. To do that, you only have to change one line of
printMultTable. Change

As an exercise, trace the execution of this version of
printMultTable and figure out how it works.

6.9 Functions

A few times now, we have mentioned "all the things functions are good
for." By now, you might be wondering what exactly those things are.
Here are some of them:

Giving a name to a sequence of statements makes your program
easier to read and debug.

Dividing a long program into functions allows you to separate parts of
the program, debug them in isolation, and then compose them into a whole.

Functions facilitate both recursion and iteration.

Well-designed functions are often useful for many programs. Once you
write and debug one, you can reuse it.

6.10 Glossary

multiple assignment

Making more than one assignment to the same
variable during the execution of a program.

iteration

Repeated execution of a set of statements using
either a recursive function call or a loop.

loop

A statement or group of statements that execute repeatedly until
a terminating condition is satisfied.

infinite loop

A loop in which the terminating condition is
never satisfied.

body

The statements inside a loop.

loop variable

A variable used as part of the terminating
condition of a loop.

tab

A special character that causes the cursor to move to
the next tab stop on the current line.

newline

A special character that causes the cursor to move to the
beginning of the next line.

cursor

An invisible marker that keeps track of where the next
character will be printed.

escape sequence

An escape character (\) followed by one or
more printable characters used to designate a nonprintable character.

encapsulate

To divide a large complex program into components
(like functions) and isolate the components from each other (by
using local variables, for example).

generalize

To replace something unnecessarily specific (like a constant
value) with something appropriately general (like a variable or parameter).
Generalization makes code more versatile, more likely to be reused, and
sometimes even easier to write.

development plan

A process for developing a program. In this chapter,
we demonstrated a style of development based on developing code to do
simple, specific things and then encapsulating and generalizing.